Extension that allows you to set a default zoom in Firefox as opposed to 100% that is the default value
The extension can be obtained here https://addons.mozilla.org/en-US/firefox/addon/fixed-zoom/
It does handle the zoom but Firefox doesn't have the best zoom API so there are limitations in trying to emulate regular zoom behavior.
In case the extension doesn't work as it should there's another one that handles the zoom with very good support: Zoom Page WE https://addons.mozilla.org/en-US/firefox/addon/zoom-page-we/